The present embodiments herein relate to transistor structures such as diodes and bipolar transistors, and more specifically, to diodes and bipolar transistors that utilize semispherical structures.
PN (P-type region/N-type region) junction diodes and PIN (P-type region-intrinsic-N-type region) diodes are commonly used in a wide variety of products such as Radio Frequency (RF) switching devices, telecommunication products, electrostatic discharge (ESD) protect devices, imaging sensors, and the like. The application of PIN diodes may also be extended to photodetectors or optical receivers for emerging optical interconnect technology. For example, when light is illuminated on a PIN photodiode, a current may be generated in the PIN photodiode based on the intensity of the light. When no light is present, the PIN photodiode may be reverse biased, and almost no current may be generated in the PIN photodiode. Therefore, PIN photodiodes are capable of detecting optical signals. Another application of PN and PIN diodes are space applications where it is highly desired to detect cosmic radiation.
A conventional PN diode is typically formed on a planar surface of a semiconductor substrate by, for example, forming an N-type layer on a P-type substrate, or a P-type layer on an N-substrate. An additional lightly doped layer or an intrinsic layer is formed between the N and P layer in the case of forming a PIN diode. However, the conventional planar PN or PIN diodes have several disadvantages. In recent years, the need to remain cost and performance competitive in the production of semiconductor devices has resulted in increasing device density in integrated circuits. To facilitate the increase in device density, the feature size of semiconductor devices continues to be reduced. In the case of the planar PN and PIN diodes, reducing feature size results in a limited junction area between the p, i, and n layers, thereby reducing the sensitivity of the PIN diode to light, for example. Another disadvantage of laterally formed PN and PIN diodes is that those diodes are sensitive to the direction of incoming light. For example, the sensitivity of laterally formed PN and PIN diode is impacted by the direction of the incoming light beam. A planar diode may have different sensitivity on lights with same density but different incoming directions.
One solution to achieve reduced feature size without sacrificing PIN diode sensitivity is forming vertical PIN diodes in deep trenches. While forming PIN diodes in deep trenches enhances junction area, and therefore diode sensitivity, the process for forming the PIN diodes is relatively complicated and costly.
Further bipolar junction transistors (BJT's) are ubiquitous in high-speed and/or high-current driver applications. Current drive in standard planar BJT's is limited by the surface area of the emitter, and the capacitive loading that is present in the alternative structure of emitter-down (or buried)/collector-up (as top layer).